Heart Rate Variability (HRV) has been widely used as a scientific measurement for monitoring the physiology of both human and animal subjects. HRV is the physiological characteristic of the variation in timing between heartbeats. The heartbeat originates in specialized tissue in the heart called the sino-atrial (SA), continuously generating an electrical impulse that spreads throughout the heart muscle. This initiates process of heart muscle contraction, a well-synchronized pump that sequentially constricts all 4 chambers of the heart (two atria and two ventricles.)
The SA node signals (approximately 100-120 impulses per minute when the heart is at rest) are regulated by the autonomic nervous system (ANS) by inhibiting some of the electrical impulses. The net effect results in a normal resting heart rate (in healthy individuals) of about 55 to 70 beats per minute (at rest). This autonomic nervous system is that part of the nervous system that is not under conscious control. It controls the organs and systems of the body that are rhythmic, regular and automatic such as breathing, digestion and heart rate. There are two branches of the autonomic nervous system: sympathetic and parasympathetic.
The sympathetic nervous system provides the basal heartbeat (HB) rhythm based on overall need. This response of the heart rate to normally encountered levels of sympathetic stimulation is modulated by parasympathetic stimulation. This heartbeat response to the parasympathetic nervous system, in contrast to the sympathetic nervous system, occurs rapidly and frequently. The deceleration of the heartbeat is almost instantaneous. It only takes 1 or 2 heartbeats to see these changes take place, slowing the heart rate.
HRV analysis can be used in both clinical and non-clinical applications for a diverse range of evaluations. In healthy individuals, the HR is variable. It fluctuates and generally, greater variability (or HRV) correlates with better health. Higher HRV indicates a healthy autonomic nervous system, and in particular, healthy balance between the sympathetic and parasympathetic systems. A decreased HRV is an early, accurate indicator that the autonomic nervous system is out of balance. The lower the HRV, the greater the imbalance in autonomic control and the greater the likelihood of poor health, both now and in the future.
Clinical applications for HRV analysis are related to cardiac health, and are indications that are shown to directly relate to health changes with many chronic and critical health conditions. Included are, but not limited to, risk of a cardiac event, occurrence of diabetes, episodic and chronic mental health conditions, sleep apnea, SIDS, exposure to and incidence of allergic reactions.
In non-clinical applications, it has been shown that HRV is effective in indicating a variety of physiological conditions. During vigorous exercise, HRV has been shown to be a marker for entering lactate threshold or anaerobic metabolism. Further, it is shown to be an indicator of physical fatigue, exercise capacity, endurance, and overall fitness. Application has been found to be useful in assessing physiological-behavioral conditions, such as stress in trainee stock market traders and driver fatigue.
There are several ways to measure and analyze HRV. Heart rate signals are obtained through electrocardiogram (ECG) or by pulse wave measurement called “Photoplethysmography” (PPG). The most accurate clinical determination of HRV is derived from measuring the duration of the intervals between contractions of the heart, called interbeat intervals, on ECG (or EKG). In contrast, PPG is less invasive, simpler to apply, and can conveniently access capillaries in a fingertip or the earlobe. Using differential light absorption characteristics and an optical sensor, PPG detects changes in the pulse waves generated by blood flow through the microcirculation. In this way an accurate estimate of HRV can be obtained.
The present invention features system, devices, and methods for real-time HRV monitoring. The HRV systems and devices of the present invention are adapted to give immediate feedback to the subject concerning their current physiological condition and any pertinent changes in their physiology.
A few studies that outline some applications of HRV benefiting from real-time feedback include, but are not limited to clinical applications with real time relevance such as anticipation of mood changes in patients with Bipolar Disorder, alerting the onset of infant physiological dysfunction during sleep, early warning of epileptic seizure, food allergy alerting, and sleep apnea; and non-clinical applications with real time relevance such as predicting the onset of lactate threshold in endurance athletes, warning of physiological effects of pollution, particularly volatile organic compounds (VOCs), alerting the onset of driver fatigue, and monitoring professionals in high stress occupations (e.g., air traffic controllers). These scenarios and many others may benefit from the accurate monitoring, analysis and real time alerting, to a relevant change in physiology as indicated by a change in HRV.